When you create a double stone slab block of each type (stone, bricks, nether bricks, etc), it automatically transforms into a double stone slab block. with the F3 menu, we can also see that there is a Round Trip Different problem, with different A/E problem in each kind of slabs.
Double Slab blocks already generated (in 14w11b for example) also transform by this way.
Wooden Slab aren't affected by this bug.